Cation ratio dating
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Thorium is a weakly radioactive metallic chemical element with symbol Th and atomic number 90.
Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement.
At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).
Thorium metal has a bulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same as tin's (58.2 GPa).
The closeness in energy levels of the 5f, 6d, and 7s energy levels of thorium results in thorium almost always losing all four valence electrons and occurring in its highest possible oxidation state of 4.
This is different from its lanthanide congener cerium, in which 4 is also the highest possible state, but 3 plays an important role and is more stable.
Thorium forms eutectic mixtures with chromium and uranium, and it is completely miscible in both solid and liquid states with its lighter congener cerium.
All but two elements up to bismuth (element 83) have an isotope that is practically stable for all purposes ("classically stable"), with the exceptions being technetium and promethium (elements 43 and 61).electron configuration in the ground state, as the 5f and 6d subshells in the early actinides are very close in energy, even more so than the 4f and 5d subshells of the lanthanides: thorium's 6d subshells are lower in energy than its 5f subshells, because its 5f subshells are not well-shielded by the filled 6s and 6p subshells and are destabilised.This is due to relativistic effects, which become stronger near the bottom of the periodic table, specifically the relativistic spin–orbit interaction.Pa, and this process is often used to check the results of uranium–thorium dating.Uranium–thorium dating is commonly used to determine the age of calcium carbonate materials such as speleothem or coral, because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor sediments, where their ratios are measured.In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity.